The invention relates to improvements in hydraulic operating arrangements, particularly to operating arrangements for clutches in the power trains of motor vehicles. More particularly, the present invention relates to improvements in hydraulic operating arrangements which can be utilized with advantage in or in combination with so-called master and slave cylinders to actuate friction clutches between the prime movers (such as internal combustion engines) and the change-speed transmissions in the power trains of motor vehicles.
Published German patent application No. 197 42 468 discloses a clutch operating arrangement wherein the housing or casing of the slave cylinder consists, at least in part, of a plastic material. Such slave cylinders are known as central clutch release devices and their casings are normally affixed to the transmission housing or case by threaded fasteners. At least a part of the casing of the slave cylinder is mounted in such a way that it surrounds the input shaft of the change-speed transmission. Securing of the casing of the slave cylinder to the transmission case by threaded fasteners during assembly of the power train is time consuming and hence expensive.
Presently known seals, such as lip seals, which serve to seal the internal chamber of the casing of the slave cylinder from the atmosphere when such chamber is to receive hydraulic fluid from the master cylinder are normally mounted on the reciprocable piston in the slave cylinder of a brake actuator. Such seals share the axial movements of the piston and are called upon to move along inner and outer sealing surfaces which are respectively provided on a metallic and on a plastic material. The sealing surface which is provided on a plastic material normally offers a greater frictional resistance to axial movements of the piston and is likely to generate noise while guiding a piston during actuation (such as disengagement) of the friction clutch.
It is also known to provide the slave cylinder in the hydraulic clutch actuating system of the power train in a motor vehicle with an internal sleeve which serves to guide the piston of the slave cylinder and is made of steel or an equivalent material. A drawback of such slave cylinders is that adequate sealing of the contact surface(s) between the guide sleeve of steel and the plastic casing of the slave cylinder can create serious problems. In order to avoid such problems (including high cost or an unsatisfactory seal), the casings or cylinders of many presently known slave cylinders do not employ separately produced guide sleeves for the piston, i.e., the guide sleeve is part of the plastic casing. A drawback of such proposals is that the making of a slave cylinder casing without a separately produced guide sleeve for the piston but with an adequate guide surface for the piston (i.e., with a guide surface provided on a part made of a plastic material) cannot be made at a reasonable cost in available machines, e.g., in standard injection molding machines for the making of articles from plastic material. As a rule, an injection molding machine which is to turn out such plastic slave cylinder casings must be equipped with highly complex parts such as shifting or pushing arrangements and the like.